In oilfield industries, acoustic or sonic logging data are useful to obtain some properties of the formation or borehole. In particular, formation mobility or permeability is an important one of the properties obtained from the acoustic or sonic logging data. Formation mobility is generally defined as a ratio of permeability to viscosity. Permeability is generally a measure of the ease with which fluid can move through a porous rock. Borehole Stoneley wave data is known to be sensitive to formation mobility. U.S. Pat. No. 5,687,138, which is incorporated by reference, discloses that formation mobility is determined by using Stoneley waveforms. In short, Stoneley waveforms are analyzed with borehole fluid slowness (i.e. inverse of velocity) and borehole fluid attenuation, external parameters through multiparameter inversion to obtain formation mobility. Stoneley waveforms can be also analyzed with external parameters through multiparameter inversion to obtain borehole fluid slowness and borehole fluid attenuation.
It would be desirable to give a characterization of fluid in a formation based on acoustic or sonic data. This is because acoustic or sonic data can be acquired by acoustic or sonic tools which originally comprise simple devices with little electronics. However, at present, Nuclear Magnetic Resonance (NMR) logging technique is a typical one and well-known for a characterization of fluid. NMR logging techniques can take direct measurements of fluid (hydrogen atom(s) to be exact) in a formation. In this case, for example, NNR logging could be used to identify heavy oil in a formation based on estimation of fluid viscosity. On the other hand, NMR logging techniques are available for limited borehole conditions. Thus, other techniques for fluid characterization would be required.